Good lord, a $20 solution to a rare problem. Do you have Tupperware storage containers in these sizes? Do you own a roll of plastic wrap?
put your guacamole in the container, level off the top, then put a layer of plastic wrap down on it. Push out air pockets and pat it down all the way to the edges, and with a spoon handle tuck the outer edge down a bit between the dip and the side of the container. Seal the top.
There, now you don’t have to figure out where to put another gadget you almost never use.